Normal development requires that nerves grow to make proper contacts with other nerves or muscles. This project contributes to our knowledge of this stage of nervous system development, so that a clearer understanding of the etiology of human developmental defects arising from abnormalities of this process can be gained. As the wiring up of the nervous system occurs early in development, this process is often studied in model organisms whose embryos develop externally. In addition, as these processes have been highly conserved across the animal kingdom, model organisms amenable to genetic analysis such as fruit flies offer ideal experimental models, with knowledge gained in flies often directly applicable to humans. As a nerve cell extends its axon to connect to another nerve or muscle cell, the tip of that axon must properly navigate through three-dimensional space. Receptor proteins that extend across the tip of the axon's membrane sense both attractive and repulsive guidance cues, and the presence of those cues is translated into changes in the axon's cytoskeleton, causing the axon to extend, retract, or turn as appropriate. This project centers on the signal transduction networks linking trans-membrane guidance receptors to cytoskeletal dynamics, focusing on two key players in these networks, the Abl kinase protein and the Trio protein. Previous work in fruit flies has shown these two to have a strong genetic interaction, and Specific Aim 1 of the research is to determine the detailed biochemical underpinnings of that genetic interaction. The application's two other aims use the power of fruit fly genetics as a tool of discovery, as one way to uncover novel biochemical interactions is to find mutations that have synergistic interactions. A novel genetic mutation that exacerbates the trio mutant phenotype has been discovered, and the specific gene responsible for this effect will be cloned and characterized. Also, a systematic search encompassing more than half of the fruit fly genome will be undertaken to identify and catalog other, novel mutations with the ability to modify either the trio or the abl mutant phenotypes. Through these aims a more complete picture of the proteins involved in linking axon guidance receptors to cytoskeletal dynamics will be gained. A fuller comprehension of the signals required for normal axon guidance will set the stage for an understanding of abnormalities that may arise in these critically important processes.